This is a little expansion on what I wrote as a comment in my previous entry on Ultraman Nexus. For spoiler reason, I would mark out names whenever I outright state them (most of the time, only when it might not be clear who I'm talking about). Most of the time I consider a couple canon only if there seem to be real compelling evidences that the show creators like me to think so. That said, there is no guarantee of accuracy of this list since I'm mostly going by memory, and I haven't seen some of these series recently.

Ultraman Tiga - I think there are at least two canon couples. And both of those are not ambiguous at all.

Ultraman Dyna - Not sure, but probably at least one.

Ultraman Gaia - I think at least two, though I'm wibbling on one, since it seems to be canon only much later.

Ultraman Neos - I've forgotten almost everything about this show (in fact, I pretty much forgot its existence until recently), so I'll have to pass on this one.

Ultraman Cosmos - Seems like there is none (well, maybe there are hints of Musashi & Ayano), but I'm not too surprised since this is probably the most kid-friendly show.

Ultraman Nexus - I count at least five. Three of them involve only four people, however. Since I didn't bother listing them previously, I might as well do it this time.

In no real particular order: Komon and Riko, Mizorogi & Nagi, Komon and Nagi, Himeya & Sakuta (admit that I might be delusional about this one), Ren and Mizuo (the cutest couple in this series, imho).

Ultraman Max - I think there are at least two, maybe. Kaito & Mizuki is totally obvious since day one, and hopefully I'm not the only one who sees anything between Koba and Elly...

Ultraman Mebius - I don't really see much, but maybe one I can say pretty much for sure (Sakomizu & Misaki), and another one I think only if viewers want to see it (George & Marina). In general, though, I think Mebius is probably not the right series to come looking for canon couples...

UltraSeven X - I remember only one. There may be more, however...

Ultra Galaxy - I can't think of any (nope, don't feel there is enough between Rei and Haruna), and I can't even use the same reason I use for Cosmos...

Open to corrections different opinions (and corrections on any misspelling on names, since I'm a bit tired to go look them up). But yeah, my definition of a canon couple might be too strict...